The present invention relates to a discriminating device for discriminating between two kinds of objects based on surface reflectivity differences thereof, and more specifically to a device for discriminating between printing-plate and interleaf paper.
In CTP devices for direct imaging on printing plates, the plates are accommodated in a magazine, or cassette and provided one by one to be exposed by the imaging device.
The plates provided in the cassette are usually separated by interleaf paper, interposed between the plates, to prevent friction damages to the plates"" emulsion-covered surfaces.
In the course of imaging plates, the plate placed at the top of the stack is picked and transferred to the exposure area for imaging. If an interleaf paper (slip-sheet) is placed at the top of the stack, the paper is picked and disposed of.
Accordingly, discriminating means for discriminating between plate and paper are used, to correctly identify the topmost object on the stack.
Published application EP 1136403 describes a discriminating device comprising two optical sensors. One of the sensors reacts to light reflected by both plate and paper, while the second sensor reacts only to light reflected by a plate. The first and the second sensors are placed on the optical axis of the light reflected from the sensed surface. Both sensors react to a predetermined intensity of reflected light. The difference is that the second sensor is inclined and its predetermined intensity is lower, in order to react to polymer surface only.
The disadvantage of the method of EP 1136403 lies in its low discrimination factor (xe2x80x98paperxe2x80x99 to xe2x80x98metalxe2x80x99 signal ratio), which would require intensity (threshold) adjustments per batch of plates of the same type having different reflectivity, on top of intensity adjustment per plate-type.
There is need for a discriminating device that overcomes the shortcomings of existing devices. The required device should be independent of variations in material and of sub-variations within plates of the same material.
The proposed surface discriminating method and device are free of the drawbacks of existing solutions, due to extended ratio of plate sensing to paper sensing signals, covering all differences between different materials and batches of the same material.
According to one aspect of the present invention, there is provided an apparatus for discriminating between objects having different surface reflectivity, said objects positioned in one of a plurality of angles relative to said apparatus, comprising:
a light source for illuminating an object;
a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said object in a first one of said plurality of angles;
a second optical sensor mounted relative to said light source, said second optical sensor receiving direct light reflected from said object in a second one of said plurality of angles;
a third optical sensor mounted relative to said light source, said third optical sensor receiving dispersed light reflected from said object in any of said plurality of angles;
adding means for computing the sum of a light signal received by said first sensor and a light signal received by said second sensor;
computation means for comparing a light signal received by said third optical sensor with said sum, said computation means connected with said adding means and with said third optical sensor; and
means for determining said object type, said means for determining connected with said computation means.
In one embodiment, the computation means comprise means for subtracting.
The apparatus may additionally comprise a first amplifier connected with said adding means and a second amplifier connected with said third optical sensor, wherein said first amplifier and said second amplifier are connected with said computation means.
The means for determining may comprise a comparator and a reference, wherein said comparator compares an input thereof, received from said computation means, with said reference.
The apparatus may additionally comprise a third amplifier connected with said computation means and with said means for determining.
According to another aspect of the present invention, there is provided a CTP device for imaging printing plates stacked in a cassette, said plates separated by paper sheets, said cassette positioned in one of a plurality of angles, comprising:
an imaging system;
discriminating means for determining the type of a topmost object in said cassette, said discriminating means comprising:
a light source for illuminating said topmost object;
a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said topmost object in a first one of said plurality of angles;
a second optical sensor mounted relative to said light source, said second optical sensor receiving direct light reflected from said topmost object in a second one of said plurality of angles;
a third optical sensor mounted relative to said light source, said third optical sensor receiving dispersed light reflected from said topmost object in any of said plurality of angles;
adding means for computing the sum of a light signal received by said first sensor and a light signal received by said second sensor;
computation means for comparing a light signal received by said third optical sensor with said sum, said computation means connected with said adding means and with said third optical sensor; and
means for determining said topmost object type, said means for determining connected with said computation means.
In one embodiment, the computation means comprise means for subtracting.
The apparatus may additionally comprise a first amplifier connected with said adding means and a second amplifier connected with said third optical sensor, wherein said first amplifier and said second amplifier are connected with said computation means.
The means for determining may comprise a comparator and a reference, wherein said comparator compares an input thereof, received from said computation means, with said reference.
The apparatus may additionally comprise a third amplifier connected with said computation means and with said means for determining.
In one embodiment, the comparator""s output is a logic high when said topmost object is a plate and a logic low when said topmost object is a paper sheet.
In another embodiment, the comparator""s output is a logic low when said topmost object is a plate and a logic high when said topmost object is a paper sheet.
In yet another aspect of the present invention, there is provided a method of discriminating between objects having different surface reflectivity, said objects positioned in one of a plurality of angles, comprising the steps of:
providing a light source for illuminating an object;
providing a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said object in a first one of said plurality of angles;
providing a second optical sensor mounted relative to said light source, said second optical sensor receiving direct light reflected from said object in a second one of said plurality of angles;
providing a third optical sensor mounted relative to said light source, said third optical sensor receiving dispersed light reflected from said object in any one of said plurality of angles;
comparing a light signal received by said third optical sensor with a sum of the light signals received by said first and second optical sensors; and
determining said object type based on said step of comparing.
In one embodiment, the step of comparing comprises subtracting.
In yet another aspect of the present invention, there is provided, in a CTP machine, a method of determining the type of a topmost object in a plate loading cassette, said cassette positioned in one of a plurality of angles, comprising the steps of:
providing a light source for illuminating said topmost object;
providing a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said topmost object in a first one of said plurality of angles;
providing a second optical sensor mounted relative to said light source, said second optical sensor receiving direct light reflected from said topmost object in a second one of said plurality of angles;
providing a third optical sensor mounted relative to said light source, said third optical sensor receiving dispersed light reflected from said topmost object in any one of said plurality of angles;
comparing a light signal received by said third optical sensor with a sum of the light signal received by said first and second optical sensors; and
determining said topmost object type based on said step of comparing.
In one embodiment, the step of comparing comprises subtracting.